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A '''Job plot''' (also known as the '''method of continuous variation''' or '''Job's method'''; named after P. Job) is used to determine the [[stoichiometry]] of a binding event. This method is widely used in [[analytical chemistry]], [[instrumental analysis]], and advanced [[chemical equilibrium]] texts and research articles.
In 1928, Paul Job published a paper entitled Formation and Stability of Inorganic Complexes in Solution. He looked into the study of ion associations. Part of his research focused on relating UV absorption to the mole fraction of {{chem2|Tl(NO3)}} in a {{chem2|Tl(NO3)/NH3}} solution. <ref>Job, P. ''Annali di Chimica Applicata'' (1928) '''9''', 113-203</ref> Plotting the absorbance versus mole fraction yielded the first Job Plot, which provided Job with information about equilibrium complexes and thus complexes present in solution.
In solutions where two species are present (i.e. species A and species B), one species (A) may bind to the other species (B). In some cases, more than one A will bind with a single B. One way to determine the amount of A binding to B is by using a Job plot.
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[[File:Job Plot 1.jpg|thumb|A simple Job Plot showing how a physical property changes upon changing the mole fraction of compound A.]]
χ<sub>A</sub> is the mole fraction of compound A and P is the physical property being measured to understand complex formation. This property is most often times UV absorbance. <ref>Renny, J. S.; Tomasevich, L. L.; Tallmadge, E. H.; Collum, D. B. Method of Continuous Variations: applications of job plots to the molecular associations in organometallic chemistry. ''Angew Chem Int Ed Engl'' (2013) '''46''', 11998 – 2013. </ref>
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